Desynchronization of internal circadian rhythms, induced by shifts of the normal sleep-wake cycle, irregular work patterns or various disease conditions, profoundly affects performance, increases the risk of accident and promotes cardiovascular and psychiatric disorders. Progress towards ameliorating the debilitating effects of circadian desynchrony lies in understanding the neurologic basis for the regulation of circadian rhythms. The overarching goal of the proposed research is to understand the neurotransmitter systems that regulate the circadian clock located in the suprachiasmatic nuclei (SCN). Based upon anatomical, pharmacological and behavioral studies, neuropeptide Y (NPY) is thought to play critical roles in regulating the circadian timing system. These roles include the behavioral control of circadian rhythm phase, and the modulation of photic entrainment. To date, however, even the most fundamental aspects of NPY regulation remain speculative, because methodologies have not existed for measuring the in vivo release of NPY in the SCN. Thus, functional relationships between factors affecting NPY release and circadian clock entrainment are not understood. The proposed studies, incorporating the newly-validated microdialysis-mass spectrometric technique for measuring SCN NPYrelease from freely-behaving animals, will provide the first-ever information on critical aspects of NPY regulation in the circadian system. The effects of photic, behavioral and circadian influences on SCN NPY release will be evaluated. Moreover, the neurochemical regulation of NPY neurons will be characterized. This information will provide the groundwork necessary for unraveling the complex, intertwined roles of NPY, serotonin, glutamate and other neuropeptides in circadian clock regulation. Relevance of the research: This study will extend knowledge of the neurologic mechanisms that control human circadian timing, including those implicated in sleep disorders and associated diseases. The ultimate goal of this work is to aid in the development of strategies for manipulating circadian clock mechanisms to treat these debilitating conditions.